304UM0001 Rev 00
PD160 Enhanced Series User Manual
Page: 20 of 39
4. PRINCIPAL OF OPERATION
The inductive loop vehicle detector senses the presence of a vehicle over an area defined by a loop of
two or more turns of wire, laid under the road or pavement surface. This loop of wire is connected to
the detector by a twisted pair of wires called a loop feeder.
A vehicle passing over a sensing loop causes a small reduction in the inductance of the loop, which is
sensed by the detector. The sensitivity of the detector is adjustable to accommodate a wide range of
vehicle types, as well as different loop and feeder combinations.
Upon detection of a vehicle passing over the loop the detector operates its output relays, which may
be used to indicate controls associated with the installation.
4.1 Detector Tuning
Tuning of the detector is fully automatic. The detector will re-tune if any of the following events occur:
-
When power is applied to the detector
-
A channel reset is initiated via the menu system.
-
A detect of greater than 16%
∆
L/L occurs.
-
A fault has occurred and is self-healing.
The detector will automatically tune to any loop with an inductance in the range 20 to 1500 micro-
henries (
µ
H).
This wide range ensures that all loop sizes and feeder combinations will be accommodated in the
tuning range of the detector.
Once tuned, any slow environmental change in loop inductance is fed to a compensating circuit within
the detector, which keeps the detector correctly tuned.
For more information about tuning, noise and drift refer to section 3.2.2.1.
For more information about diagnostics, refer to section 3.2.6.
4.2 Detector Sensitivity
Sensitivity of the detection system is dependent on factors such as loop size, number of turns in the
loop, feeder length and the presence of metal reinforcing beneath the loop.
The nature of the application determines the required sensitivity, which may be adjusted by means of
the LCD Menu system on the front of the enclosure.
